diff --git a/packages/core/postgrest-js/src/PostgrestFilterBuilder.ts b/packages/core/postgrest-js/src/PostgrestFilterBuilder.ts
index cb53017f4c..c5d099a262 100644
--- a/packages/core/postgrest-js/src/PostgrestFilterBuilder.ts
+++ b/packages/core/postgrest-js/src/PostgrestFilterBuilder.ts
@@ -35,6 +35,24 @@ export type IsStringOperator<Path extends string> = Path extends `${string}->>${
 
 const PostgrestReservedCharsRegexp = new RegExp('[,()]')
 
+/**
+ * Serialize a single value for an `in.(...)` / `not.in.(...)` filter list.
+ *
+ * PostgREST treats `,`, `(` and `)` as reserved characters inside the list, so
+ * any value containing one must be wrapped in double quotes. A quoted value may
+ * itself contain `"` or `\`, which PostgREST expects to be backslash-escaped
+ * (`"` -> `\"`, `\` -> `\\`). Without escaping, a value such as `a"b,c`
+ * serializes to `"a"b,c"`, whose stray quote corrupts the parsed list.
+ */
+function formatInFilterValue(value: unknown): string {
+  const str = `${value}`
+  if (typeof value === 'string' && (PostgrestReservedCharsRegexp.test(str) || /["\\]/.test(str))) {
+    // Escape backslashes first, then double quotes, then wrap in quotes.
+    return `"${str.replace(/\\/g, '\\\\').replace(/"/g, '\\"')}"`
+  }
+  return str
+}
+
 // Match relationship filters with `table.column` syntax and resolve underlying
 // column value. If not matched, fallback to generic type.
 // TODO: Validate the relationship itself ala select-query-parser. Currently we
@@ -835,14 +853,7 @@ export default class PostgrestFilterBuilder<
             never
     >
   ): this {
-    const cleanedValues = Array.from(new Set(values))
-      .map((s) => {
-        // handle postgrest reserved characters
-        // https://postgrest.org/en/v7.0.0/api.html#reserved-characters
-        if (typeof s === 'string' && PostgrestReservedCharsRegexp.test(s)) return `"${s}"`
-        else return `${s}`
-      })
-      .join(',')
+    const cleanedValues = Array.from(new Set(values)).map(formatInFilterValue).join(',')
     this.url.searchParams.append(column, `in.(${cleanedValues})`)
     return this
   }
@@ -863,14 +874,7 @@ export default class PostgrestFilterBuilder<
           : never
     >
   ): this {
-    const cleanedValues = Array.from(new Set(values))
-      .map((s) => {
-        // handle postgrest reserved characters
-        // https://postgrest.org/en/v7.0.0/api.html#reserved-characters
-        if (typeof s === 'string' && PostgrestReservedCharsRegexp.test(s)) return `"${s}"`
-        else return `${s}`
-      })
-      .join(',')
+    const cleanedValues = Array.from(new Set(values)).map(formatInFilterValue).join(',')
     this.url.searchParams.append(column, `not.in.(${cleanedValues})`)
     return this
   }
diff --git a/packages/core/postgrest-js/test/in-filter-escaping.test.ts b/packages/core/postgrest-js/test/in-filter-escaping.test.ts
new file mode 100644
index 0000000000..e73caca8b5
--- /dev/null
+++ b/packages/core/postgrest-js/test/in-filter-escaping.test.ts
@@ -0,0 +1,71 @@
+import { PostgrestClient } from '../src/index'
+
+const REST_URL = 'http://localhost:3000'
+
+/**
+ * Build a client whose fetch records the request URL instead of hitting a
+ * server, so we can assert exactly how `.in()` / `.notIn()` serialize values.
+ */
+function clientCapturingUrl(): { postgrest: PostgrestClient; getUrl: () => string } {
+  let captured = ''
+  const fetchMock: typeof fetch = (input: RequestInfo | URL) => {
+    captured = typeof input === 'string' ? input : input.toString()
+    return Promise.resolve(new Response('[]', { status: 200 }))
+  }
+  const postgrest = new PostgrestClient(REST_URL, { fetch: fetchMock })
+  return { postgrest, getUrl: () => captured }
+}
+
+/** Decode the value of a query parameter from a raw (percent-encoded) URL. */
+function paramValue(rawUrl: string, key: string): string {
+  const query = rawUrl.split('?')[1] ?? ''
+  for (const pair of query.split('&')) {
+    const eq = pair.indexOf('=')
+    // URLSearchParams encodes spaces as '+', which decodeURIComponent leaves as-is.
+    const decode = (s: string) => decodeURIComponent(s.replace(/\+/g, ' '))
+    const k = decode(pair.slice(0, eq))
+    if (k === key) return decode(pair.slice(eq + 1))
+  }
+  throw new Error(`param ${key} not found in ${rawUrl}`)
+}
+
+describe('in() / notIn() reserved-character escaping', () => {
+  test('quotes values containing reserved characters', async () => {
+    const { postgrest, getUrl } = clientCapturingUrl()
+    await postgrest.from('t').select().in('name', ['a,b', 'plain'])
+    expect(paramValue(getUrl(), 'name')).toBe('in.("a,b",plain)')
+  })
+
+  test('escapes an embedded double quote inside a quoted value', async () => {
+    const { postgrest, getUrl } = clientCapturingUrl()
+    // Value has both a reserved char (comma) and a double quote.
+    await postgrest.from('t').select().in('name', ['a"b,c'])
+    // The inner quote must be backslash-escaped so the quoted value stays intact.
+    expect(paramValue(getUrl(), 'name')).toBe('in.("a\\"b,c")')
+  })
+
+  test('escapes a value that contains only a double quote', async () => {
+    const { postgrest, getUrl } = clientCapturingUrl()
+    await postgrest.from('t').select().in('name', ['say "hi"'])
+    expect(paramValue(getUrl(), 'name')).toBe('in.("say \\"hi\\"")')
+  })
+
+  test('escapes backslashes inside a quoted value', async () => {
+    const { postgrest, getUrl } = clientCapturingUrl()
+    await postgrest.from('t').select().in('path', ['a\\b,c'])
+    // Backslash doubled, then wrapped in quotes.
+    expect(paramValue(getUrl(), 'path')).toBe('in.("a\\\\b,c")')
+  })
+
+  test('leaves simple values unquoted', async () => {
+    const { postgrest, getUrl } = clientCapturingUrl()
+    await postgrest.from('t').select().in('status', ['ONLINE', 'OFFLINE'])
+    expect(paramValue(getUrl(), 'status')).toBe('in.(ONLINE,OFFLINE)')
+  })
+
+  test('notIn() escapes the same way as in()', async () => {
+    const { postgrest, getUrl } = clientCapturingUrl()
+    await postgrest.from('t').select().notIn('name', ['a"b,c'])
+    expect(paramValue(getUrl(), 'name')).toBe('not.in.("a\\"b,c")')
+  })
+})